Multi-carrier modulation and demodulation system using a half-symbolized symbol

ABSTRACT

The communication apparatus at a transmission side executes an inverse Fourier transform to a signal after a BPSK modulation, thereby to generate a transmission symbol, and transmits the transmission symbol in a half-symbolized status. The communication apparatus at a reception side separates a received signal into even sub-carriers and odd sub-carriers, and first demodulates only the received symbol of the half-symbolized even sub-carriers. The communication apparatus at the reception side removes the symbol component of the even sub-carriers from the received symbol, and demodulates only the received symbol of the odd sub-carriers.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP01/08364 which has an Internationalfiling date of Sep. 26, 2001, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a communication apparatus that employsa multi-carrier modulation/demodulation system. The inventionparticularly relates to a communication apparatus and a communicationmethod capable of realizing data communications by using existingcommunication lines, based on a DMT (Discrete Multi Tone)modulation/demodulation system or an OFDM (Orthogonal Frequency DivisionMultiplex) modulation/demodulation system. However, the presentinvention is not limited to a communication apparatus that carries outdata communications based on the DMT modulation/demodulation system. Itis also possible to apply the present invention to all communicationapparatuses that carry out wire communications and radio communicationsbased on a multi-carrier modulation/demodulation system and asingle-carrier modulation/demodulation system via normal communicationlines.

BACKGROUND ART

The operation of a conventional communication apparatus will beexplained below. First, the operation of a transmission system of aconventional communication apparatus that employs the OFDMmodulation/demodulation system as a multi-carriermodulation/demodulation system will be briefly explained. When datacommunications are carried out according to the OFDMmodulation/demodulation system, the transmission system carries out atone ordering processing. In other words, the transmission systemallocates transmission data of a transmittable number of bits to aplurality of tones (multi-carriers) of a frequency band that has beenset in advance. For example, transmission data of a predetermined numberof bits is allocated to tone 0 to tone X (X is an integer that shows anumber of tones) of each frequency. The transmission data is multiplexedfor each one frame by carrying out the tone ordering processing and anencoding processing.

Further, the transmission system carries out an inverse fast Fouriertransform (IFFT) to multiplexed transmission data, and converts aparallel data after the inverse fast Fourier transform into a serialdata. Then, the transmission system converts a digital waveform into ananalog waveform with a D/A converter. Last, the transmission systemapplies a low-pass filter, and transmits the transmission data to atransmission route.

Next, the operation of a reception system of the conventionalcommunication apparatus that employs the OFDM modulation/demodulationsystem as a multi-carrier modulation/demodulation system will be brieflyexplained. When data communications are carried out according to theOFDM modulation/demodulation system, the reception system applies alow-pass filter to a received data (the above transmission data). Then,the reception system converts an analog waveform into a digital waveformwith an A/D converter, and carries out an adaptive equalization of atime domain with a time domain equalizer.

Further, the reception system converts the serial data after theadaptive equalization of the time domain into a parallel data. Thereception system carries out a fast Fourier transform to this paralleldata, and then carries out an adaptive equalization of a frequencydomain with a frequency domain equalizer.

The data after the adaptive equalization of the frequency domain isconverted into a serial data according to a composite processing (amaximum likelihood composite method) and a tone ordering processing.Then, a rate converting processing, an FEC (forward error correction), adescramble processing, and a CRC (cyclic redundancy check) are carriedout. Last, a transmission data is reproduced.

As explained above, the conventional communication apparatus thatemploys the OFDM modulation/demodulation system makes it possible tocarry out communications at a high transmission rate by utilizing goodtransmission efficiency and flexibility of functions that cannot beobtained according to the CDMA and the single-carriermodulation/demodulation system.

However, the conventional communication apparatus that employs the OFDMmodulation/demodulation system has had room for improvement in thetransmission system and the reception system from the viewpoint of“further improvement in the transmission rate”. There has been a problemthat the conventional communication apparatus has not realized anoptimum transmission rate by making a maximum utilization of the “goodtransmission efficiency” and the “flexibility of functions” that are thecharacteristics of the OFDM modulation/demodulation system.

Therefore, it is an object of the present invention to provide acommunication apparatus and a communication method capable of realizingfurther improvement in the transmission rate, by realizing halfsymbolization in the multi-carrier modulation/demodulation system.

DISCLOSURE OF THE INVENTION

The communication apparatus according to the present invention is astructure that employs the multi-carrier modulation/demodulation system,further comprises a transmission unit which generates a transmissionsymbol by carrying out an inverse Fourier transform to a signal after aBPSK modulation and transmits the transmission symbol in ahalf-symbolized status, and a reception unit which carries out apredetermined Fourier transform to the half-symbolized received symbolin order to extract even sub-carriers to demodulate data allocated tothe sub-carriers, carries out an inverse Fourier transform to the dataallocated to the even sub-carriers to generate a first symbol that isstructured with a time waveform of even sub-carriers, removes the firstsymbol component from the received symbol to generate a second symbolthat is structured with a time waveform of odd sub-carriers, adds asymbol obtained by copying and inverting the symbol to the back of thesecond symbol to generate a third symbol, and carries out apredetermined Fourier transform to the third symbol in order to extractodd sub-carriers to demodulate data allocated to the sub-carriers.

In a communication apparatus according to the next invention, thereception unit further carries out an inverse Fourier transform to thedata allocated to the odd sub-carriers to generate a fourth symbol thatis structured with a time waveform of odd sub-carriers, thereafter,removes the fourth symbol component from the received symbol, and thencarries out demodulation processing by using a received symbol after theremoval of the fourth symbol component.

A communication apparatus according to the next invention is a structurethat employs a multi-carrier modulation/demodulation system, furthercomprises a transmission unit which generates a transmission symbol bycarrying out an inverse Fourier transform to a signal after a BPSKmodulation and transmits the transmission symbol in a half-symbolizedstatus, and a reception unit which generates a first symbol that isstructured with time axis data of even sub-carriers and odd sub-carriersby generating all data combinations that could occur and by sequentiallycarrying out an inverse Fourier transform to the data combinations,generates a second symbol that is half-symbolized by deleting a latterhalf section of the first symbol and subtracts an occasionally-generatedsecond symbol from the received symbol, calculates a squared average (adispersion value) of amplitude by using a received subtraction resultand detects a minimum value from the squared average, and determines adata combination corresponding to the minimum value as most likely fromamong all the data combinations that could occur and decides this datacombination as a final decision value.

A communication apparatus according to the next invention is a structurethat employs a multi-carrier modulation/demodulation system, furthercomprises a transmission unit which generates a transmission symbol bycarrying out an inverse Fourier transform to a signal after a BPSKmodulation and transmits the transmission symbol in a half-symbolizedstatus, and a reception unit which generates a first symbol that has thesame length as that of the symbol before the half-symbolization byadding a symbol of all 0 to the back of a half-symbolized receivedsymbol, extracts even sub-carriers and odd sub-carriers by carrying outa Fourier transform to the first symbol, and judges individually datathat is allocated to the even sub-carriers and data that is allocated tothe odd sub-carriers.

A communication apparatus according to the next invention is a structurethat employs a multi-carrier modulation/demodulation system, and furthercomprises a reception unit which generates a transmission symbol bycarrying out an inverse Fourier transform to a signal after a QPSKmodulation and transmits the transmission symbol in a half-symbolizedstatus, and a transmission unit which generates a first symbol that hasthe same length as that of the symbol before the half-symbolization byadding a symbol of all 0 to the back of a half-symbolized receivedsymbol, extracts even sub-carriers and odd sub-carriers by carrying outa Fourier transform to the first symbol, makes a hard decision on dataof the even sub-carriers, calculates a component that becomes aninterference to the odd sub-carriers from the decision result, removesthe interference component from the odd sub-carriers after theextraction, makes a hard decision on data of odd sub-carriers after theremoval of the interference component, calculates a component thatbecomes an interference to the even sub-carriers from the decisionresult, and removes the interference component from the evensub-carriers after the extraction, thereafter, repeatedly executes theinterference component removal processing by a predetermined number oftimes, and outputs decision results of both sub-carriers as finaldecision values.

A communication method according to the next invention comprises atransmission step of generating a transmission symbol by carrying out aninverse Fourier transform to a signal after a BPSK modulation andtransmitting the transmission symbol in a half-symbolized status, aneven sub-carrier demodulating step of carrying out a predeterminedFourier transform to the half-symbolized received symbol in order toextract even sub-carriers and demodulating data allocated to thesub-carriers, a first-symbol generating step of generating a firstsymbol that is structured with a time waveform of even sub-carriers bycarrying out an inverse Fourier transform to the data allocated to theeven sub-carriers, a second-symbol generating step of generating asecond symbol that is structured with a time waveform of oddsub-carriers by removing the first symbol component from the receivedsymbol, a third-symbol generating step of generating a third symbol byadding a symbol obtained by copying and inverting the symbol to the backof the second symbol, and an odd sub-carrier demodulating step ofcarrying out a predetermined Fourier transform to the third symbol inorder to extract odd sub-carriers and demodulating data allocated to thesub-carriers.

A communication method according to the next invention further comprisesa fourth-symbol generating step of generating a fourth symbol that isstructured with a time waveform of odd sub-carriers by carrying out aninverse Fourier transform to the data allocated to the odd sub-carriers,and a removing step of removing the fourth symbol component from thereceived symbol, and the communication method thereafter carries out ademodulation processing by using a received symbol after the removal ofthe fourth symbol component.

A communication method according to the next invention comprises atransmission step of generating a transmission symbol by carrying out aninverse Fourier transform to a signal after a BPSK modulation andtransmitting the transmission symbol in a half-symbolized status, afirst-symbol generating step of generating a first symbol that isstructured with time axis data of even sub-carriers and odd sub-carriersby generating all data combinations that could occur and by sequentiallycarrying out an inverse Fourier transform to the data combinations, asecond-symbol generating step of generating a second symbol that ishalf-symbolized by deleting a latter half section of the first symbol, asubtracting step of subtracting an occasionally-generated second symbolfrom the received symbol, a squared-average calculating step ofcalculating a squared average (a dispersion value) of amplitude by usinga received subtraction result, a minimum-value detecting step ofdetecting a minimum value from the squared average, and a decision stepof judging a data combination corresponding to the minimum value as mostlikely from among all the data combinations that could occur anddeciding this data combination as a final decision value.

A communication method according to the next invention comprises atransmission step of generating a transmission symbol by carrying out aninverse Fourier transform to a signal after a BPSK modulation andtransmitting the transmission symbol in a half-symbolized status, afirst-symbol generating step of generating a first symbol that has thesame length as that of the symbol before the half-symbolization byadding a symbol of all 0 to the back of a half-symbolized receivedsymbol, a sub-carrier extracting step of extracting even sub-carriersand odd sub-carriers by carrying out a Fourier transform to the firstsymbol, and a decision step of judging individually data that isallocated to the even sub-carriers and data that is allocated to the oddsub-carriers.

A communication method according to the next invention comprises atransmission step of generating a transmission symbol by carrying out aninverse Fourier transform to a signal after a QPSK modulation andtransmitting the transmission symbol in a half-symbolized status, afirst-symbol generating step of generating a first symbol that has thesame length as that of the symbol before the half-symbolization byadding a symbol of all 0 to the back of a half-symbolized receivedsymbol, a sub-carrier extracting step of extracting even sub-carriersand odd sub-carriers by carrying out a Fourier transform to the firstsymbol, a first interference component calculating step of making a harddecision on data of the even sub-carriers and calculating a componentthat becomes an interference to the odd sub-carriers from the decisionresult, a first removing step of removing the interference componentfrom the odd sub-carriers after the extraction, a second interferencecomponent calculating step of making a hard decision on data of oddsub-carriers after the removal of the interference component andcalculating a component that becomes an interference to the evensub-carriers from the decision result, and a second removing step ofremoving the interference component from the even sub-carriers after theextraction, thereafter, the communication method executes theinterference component removal processing by a predetermined number oftimes, and outputs decision results of both sub-carriers as finaldecision values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that shows a structure of a first embodiment of thecommunication apparatus according to the present invention;

FIG. 2 is a diagram that shows an example of a total structure of atransmission system of a communication apparatus that employs a DMTmodulation/demodulation system;

FIG. 3 is a diagram that shows an example of a total structure of areception system of the communication apparatus that employs a DMTmodulation/demodulation system;

FIG. 4 shows diagrams that are structures of an encoder and a decoderthat are used in the communication apparatus according to the presentinvention;

FIG. 5 is a diagram that shows an example of a structure of a turboencoder;

FIG. 6 shows diagrams that are waveforms of sub-carriers and combinedwaveforms respectively;

FIG. 7 is a diagram that shows a structure of a second embodiment of thecommunication apparatus according to the present invention;

FIG. 8 is a diagram that shows a structure of a third embodiment of thecommunication apparatus according to the present invention; and

FIG. 9 is a diagram that shows a structure of a fourth embodiment of thecommunication apparatus according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the communication apparatus and the communication methodaccording to the present invention will be explained in detail belowwith reference to the drawings. It should be noted that the presentinvention is not limited by these embodiments.

First Embodiment

FIG. 1 is a diagram that shows a structure of a first embodiment of thecommunication apparatus according to the present invention.Specifically, FIG. 1 is a diagram that shows a structure of a receptionside that is a characteristic of the present embodiment.

The communication apparatus of the present embodiment has bothstructures of the transmission side and the reception side. Further, thecommunication apparatus has a high-precision data correction capacityaccording to a turbo encoder and a turbo decoder, thereby to obtainexcellent transmission characteristics in data communications and soundcommunications. The present embodiment has both of the above structuresfor the sake of convenience of explanation. It is also possible toassume a transmitter that has only a structure of a transmission side,or a receiver that has only a structure of a reception side.

For example, in the structure of the reception side in FIG. 1, areference numeral 1 denotes a subtractor, and 2 denotes a fast Fouriertransforming section (128 complex FFT) that extracts only 64 evensub-carriers from among 128 sub-carriers. A reference numeral 3 denotesa decoder that decodes even sub-carriers, and 4 denotes an inverse fastFourier transforming section (128 complex IFFT) that inversely performsfast Fourier transform on only 64 even sub-carriers. A reference numeral5 denotes a subtractor, 6 denotes a symbol generating section, and 7denotes a fast Fourier transforming section (256complexFFT) thatextracts only 64 odd sub-carriers, for example. A reference numeral 8denotes a decoder, 9 denotes an inverse fast Fourier transformingsection (256 complex IFFT) that inversely performs fast Fouriertransform on only 64 odd sub-carriers, and 10 denotes a symbolgenerating section.

Before explaining the operation of the transmission side and theoperation of the reception side that become the characteristics of thepresent invention, the basic operation of the communication apparatusaccording to the present invention will be briefly explained withreference to the drawings. As radio digital communication systems thatemploy a DMT (Discrete Multi Tone) modulation/demodulation system for amulti-carrier modulation/demodulation system, there are xDSLcommunication systems like an ADSL (Asymmetric Digital Subscriber Line)communication system and an HDSL (high-bit-rate Digital Subscriber Line)communication system that carry out high-speed digital communications ofa few mega bits/second by using existing telephone lines. The XDSLsystems have been standardized in the T1.413 of ANSI.

FIG. 2 is a diagram that shows an example of a total structure of thetransmission system of the communication apparatus that employs the DMTmodulation/demodulation system. Referring to FIG. 2, in the transmissionsystem, a multiplex/synch control (corresponding to MUX/SYNC CONTROL) 41multiplexes a transmission data. Cyclic redundancy checks (correspondingto CRC) 42 and 43 add error detection codes to multiplexed transmissiondata. Then, forward error correction (corresponding to SCRAM&FEC) 44 and45 carry out addition of FEC codes and a scramble processing.

There are two routes from the multiplex/synch control 41 to a toneordering 49. One is an interleaved data buffer route that includes aninterleave (INTERLEAVE) 46, and the other is a fast data buffer routethat does not include the interleave. The interleaved data buffer routethat carries out an interleave processing has a larger delay.

Then, rate converters (corresponding to RATE-CONVERTOR) 47 and 48 carryout a rate conversion processing to the transmission data, and the toneordering (corresponding to TONE ORDERING) 49 carries out a tone orderingprocessing to the transmission data. A constellation encoder/gainscaling (corresponding to CONSTELLATION AND GAINS CALLING) 50 prepares aconstellation data (including a turbo decoding), based on thetransmission data after the tone ordering processing. An inverse fastFourier transforming section (corresponding to IFFT) 51 carries out aninverse fast Fourier transform.

Last, an input parallel/serial buffer (corresponding to INPUTPARALLEL/SERIAL BUFFER) 52 converts a parallel data after the Fouriertransform into a serial data. An analog processing/digital-to-analogconverter (corresponding to ANALOG PROCESSING AND DAC) 53 converts adigital waveform into an analog waveform, executes a filteringprocessing, and transmits the transmission data to a telephone line.

FIG. 3 is a diagram that shows an example of a total structure of thereception system of the communication apparatus that employs the DMTmodulation/demodulation system. Referring to FIG. 3, in the receptionsystem, an analog processing/analog-to-digital converter (correspondingto ANALOG PROCESSING AND ADC in the drawing) 141 executes a filteringprocessing to a received data (the above transmission data), andconverts an analog waveform into a digital waveform. A time domainequalizer (corresponding to TEQ) 142 carries out an adaptiveequalization processing of a time domain.

An input serial/parallel buffer (corresponding to INPUT SERIAL/PARALLELBUFFER) 143 converts the serial data after the adaptive equalization ofthe time domain into a parallel data. A fast Fourier transformingsection (corresponding to FFT) 144 carries out a fast Fourier transformto the parallel data. Then, a frequency domain equalizer (correspondingto FEQ) 145 carries out an adaptive equalization of a frequency domain.

A constellation decoder/gain scaling (corresponding to CONSTELLATIONDECODER AND GAIN SCALLNG) 146 and a tone ordering (corresponding to TONEORDERING) 147 carry out a decoding (turbo decoding) and a tone orderingprocessing to the data after the execution of the adaptive equalizationof the frequency domain, thereby to convert the data into a serial data.Then, rate converters (corresponding to RATE-CONVERTOR) 148 and 149carry out a rate conversion. A deinterleaver (corresponding toDEINTERLEAVE) 150 carries out a deinterleave processing. Forward errorcorrection (corresponding to DESCRAM&FEC) 151 and 152 carry out an FECprocessing and a descramble processing. Cyclic redundancy check(corresponding to CRC) 153 and 154 carry out a cyclic redundancy check.Last, a multiplex/sync control (corresponding to MUX/SYNC CONTROL) 155reproduces the received data.

The above communication apparatus has two routes in the reception systemand the transmission system respectively. It is possible to realize datacommunications with small transmission delay and at a high transmissionrate by differentially using the two routes or by simultaneouslyoperating the two routes.

The operation of the wire digital communication system that employs theDMT modulation/demodulation system for a multi-carriermodulation/demodulation system has been explained, for the sake ofconvenience of explanation. However, the present invention is notlimited to the above. It is also possible to apply the structure to allcommunication apparatuses that carry out wire communications and radiocommunications based on a multi-carrier modulation/demodulation system(for example, the OFDM modulation/demodulation system). Thecommunication apparatus that employs a turbo code has been explained asan encoding processing. However, the code is not limited to the above,and it is also possible to employ a known convolution code, for example.In the present embodiment, the time domain equalizer 142 corresponds toTEQ2 in FIG. 1, the input serial/parallel buffer 143 and the fastFourier transforming section 144 correspond to a 128 complex FFT 3 inFIG. 1, the frequency domain equalizer 145 corresponds to FEQ4 in FIG.1, and the subsequent circuits correspond to a decoder 5.

The operation of the encoder (the transmission system) and the decoder(the reception system) in the above communication apparatus that employsthe multi-carrier modulation/demodulation system will be explained belowwith reference to the drawings. FIGS. 4(a) and 4(b) are diagrams thatshow structures of an encoder (a turbo encoder) and a decoder (acombination of a turbo decoder, a hard decision unit, and an R/S(Reed-Solomon code) decoder that are used in the communication apparatusaccording to the present invention. Specifically, FIG. 4(a) is a diagramthat shows a structure of the encoder in the present embodiment, andFIG. 4(b) is a diagram that shows a structure of the decoder in thepresent embodiment.

For example, in the encoder shown in FIG. 4(a), a reference numeral 21denotes a turbo encoder that can obtain performance close to a Shannonlimit by employing a turbo code as an error correction code. For aninput of two information bits, the turbo encoder 21 outputs twoinformation bits and two redundant bits. Further, each redundant bit isgenerated such that the correction capacity of each information bitbecomes uniform at the reception side.

On the other hand, in the decoder shown in FIG. 4(b) a reference numeral22 denotes a first decoder that calculates a logarithmic likelihoodratio from a received signal Lcy (corresponding to received signals y₂,y₁, and y_(a) to be described later), 23 and 27 denote adders, and 24and 25 denote interleavers. A reference numeral 26 denotes a seconddecoder that calculates a logarithmic likelihood ratio from a receivedsignal Lcy (corresponding to received signals y₂, y₁, and y_(b) to bedescribed later), and 28 denotes a deinterleaver. A reference numeral 29denotes a first decision unit which decides an output of the firstdecoder 22, and outputs an estimated value of an original informationbit string. A reference numeral 30 denotes a first R/S decoder thatdecodes Reed-Solomon code, and outputs a higher-precision informationbit string. A reference numeral 31 denotes a second decision unit whichdecides an output of the second decoder 26, and outputs an estimatedvalue of an original information bit string. A reference numeral 32denotes a second R/S decoder that decodes Reed-Solomon code, and outputsa higher-precision information bit string. A reference numeral 33denotes a third decision unit which makes a hard decision on Lcy(corresponding to received signals y₃, y₄, . . . to be described later),and outputs an estimated value of an original information bit string.

First the operation of the encoder shown in FIG. 4(a) will be explained.In the present embodiment, a 16 QAM will be employed for a quadratureamplitude modulation (QAM). Further, in the present embodiment, theencoder executes a turbo encoding to only the lower two bits of theinput data, and outputs other higher bits in the status of the inputdata. In other words, in the present embodiment, a turbo encoding ofexcellent error correction capacity is executed to the lower two bits offour signal points (that is, four points of shortest distance betweensignal points) that have a possibility of degradation incharacteristics. A soft decision is made at the reception side. On theother hand, other higher bits that have a lower possibility ofdegradation in characteristics are output as they are, and a harddecision is made at the reception side.

Next, one example of the operation of the turbo encoder 21 shown in FIG.4(a) that executes a turbo encoding to the lower two bits u₁ and u₂ ofthe input transmission data will be explained. FIG. 5 is a diagram thatshows an example of a structure of the turbo encoder 21. It is notedthat a known recursive organization convolutional encoder will be usedas a structure of a recursive organization convolutional encoder.

In FIG. 5, a reference numeral 35 denotes a first recursive organizationconvolutional encoder that convolutionally encodes the transmission datau₁ and u₂ corresponding to the information bit string, and outputs aredundant data u_(a). Reference numerals 36 and 37 denote interleavers,and 38 denotes a second recursive organization convolutional encoderthat convolutionally encodes interleaved data u_(1t) and u_(2t), andoutputs a redundant data u_(b). The turbo encoder 21 outputssimultaneously the transmission data u₁ and u₂, the redundant data u_(a)that has been obtained by encoding the transmission data u₁ and u₂ basedon the processing of the first recursive organization convolutionalencoder 35, and the redundant data u_(b) that has been obtained byencoding (at a different time from that for the other data) theinterleaved data u_(1t) and u_(2t) based on the processing of the secondrecursive organization convolutional encoder 38.

The turbo encoder 21 avoids the occurrence of a deviation in the weightof each redundant bit so that the estimate precision of the transmissiondata u₁ and u₂ at the reception side that uses the redundant data u_(a)and u_(b) becomes uniform.

As explained above, when the encoder shown in FIG. 4(a) is used, itbecomes possible to improve the error correction capacity which correctsa burst error of data, as an effect of the interleaving. Further, itbecomes possible to make uniform the estimate precision of thetransmission data u₁ and u₂ at the reception side by changing over theinput of the transmission data u₁ series and the transmission data u₂series between the first recursive organization convolutional encoder 35and the second recursive organization convolutional encoder 38.

The operation of the decoder shown in FIG. 4(b) will be explained next.This will be explained based on the employment of the 16 QAM method forthe quadrature amplitude modulation (QAM) in the present embodiment.Further, in the present embodiment, the decoder executes a turbodecoding to the lower two bits of the received data, and estimates theoriginal transmission data based on a soft decision. The third decisionunit 33 makes a hard decision on other higher bits of the received data,and estimates the original transmission data based on the hard decision.Received signals Lcy: y₄, y₃, y₂, y₁, y_(a), and y_(b) denote signalsthat have given influence of noise and fading in the transmission routeto outputs u₄, u₃, u₂, u₁, u_(a), and u_(b) at the transmission siderespectively.

When the turbo decoder has received the signals Lcy: y₂, y₁, y_(a), andy_(b), the first decoder 22 extracts the received signals Lcy: y₂, y₁,and y_(a), and calculates logarithmic likelihood ratios L (u_(1k)′) andL (u_(2k)′) of information bits (corresponding to the originaltransmission data u_(1k) and u_(2k)) u_(1k), and u_(2k), that areestimated from these received signals (k represents a time). In otherwords, a probability that u_(2k) is 1 to a probability that u_(2k) is 0,and a probability that u_(1k) is 1 to a probability that u_(1k) is 0,are obtained. In the following explanation, u_(1k) and u_(2k) will besimply called u_(k), and u_(1k)′ and u_(2k)′ will be simply calledu_(k′.)

Where, in FIG. 4(b), Le (u_(k)) denotes external information, and La(u_(k)) denote a priori information that is external information of onebefore. As a decoder that calculates a logarithmic likelihood ratio, aknown maximum posteriori probability decoder (MAP algorithm: maximumA-posteriori) is used in many cases. Instead, a known Viterbi decodermay be used.

Next, the adder 23 calculates external information Le (u_(k)) to thesecond decoder 26, from the logarithmic likelihood ratio as a result ofthe calculation. La (uk)=0, as a priori information has not beenobtained in the first decoding.

Next, the interleavers 24 and 25 rearrange the received signal Lcy andthe external information Le (u_(k)). The second decoder 26 calculates alogarithmic likelihood ratio L (u_(k)′) based on the received signal Lcyand the a priori information La (u_(k)) that has been calculated inadvance, in a similar manner to that of the first decoder 22.

The adder 27 then calculates the external information Le (u_(k)), in asimilar manner to that of the adder 23. At this time, the externalinformation that has been rearranged by the deinterleaver 28 is fed backto the first decoder 22 as the priori information La (u_(k)).

The turbo decoder executes the above processing repeatedly by apredetermined number (iteration time), thereby to calculate ahigher-precision logarithmic likelihood ratio. The first decision unit29 and the second decision unit 31 decides on the signal based on thislogarithmic likelihood ratio, thereby to estimate the originaltransmission data. Specifically, when the logarithmic likelihood ratiois “L (u_(k)′)>0”, for example, the estimate information bit u_(k)′ isjudged as 1, and when “L (u_(k)′)≦0”, the estimate information bitu_(k)′ is judged as 0. The third decision unit 33 makes a hard decisionon the received signals Lcy: y₃, y₄, . . . that are receivedsimultaneously.

Last, the first R/S decoder 30 and the second R/S decoder 32 checkerrors by using a Reed-Solomon code according to a predetermined method.The above repeated processing is finished when a decision has been madethat a specific reference has been exceeded. The first R/S decoder 30and the second R/S decoder 32 correct errors in the originaltransmission data that have been estimated by the decision units, byusing the Reed-Solomon code, and output higher-precision transmissiondata.

Methods of estimating the original transmission data by the first R/Sdecoder 30 and the second R/S decoder 32 will be explained according toexamples. There are three method examples. A first method is as follows.Each time when the first decision unit 29 or the second decision unit 31estimates the original transmission data, the corresponding first R/Sdecoder 30 or the second R/S decoder 32 alternately checks errors. Whenany one of these R/S decoders has decided that “there is no error”, therepetitive processing by the turbo encoder is finished. Then, the errorcorrection is carried out for the estimated original transmission databy using the Reed-Solomon code, and the transmission data of higherestimate precision is output.

A second method is as follows. Each time when the first decision unit 29or the second decision unit 31 estimates the original transmission data,the corresponding first R/S decoder 30 or the second R/S decoder 32alternately checks errors. When both of these R/S decoders have decidedthat “there is no error”, the repetitive processing by the turbo encoderis finished. Then, the error correction is carried out for the estimatedoriginal transmission data by using the Reed-Solomon code, and thetransmission data of higher estimate precision is output.

A third method is as follows. This method improves a problem that thefirst and second methods make an erroneous decision that “there is noerror” and carry out an error correction when the repetitive processingis not executed. For example, in the third method, the repetitiveprocessing is executed by a predetermined number of times. After a biterror ratio has been lowered to a certain degree, the error correctionis carried out for the estimated original transmission data by using theReed-Solomon code, and the transmission data of higher estimateprecision is output.

As explained above, when the decoder shown in FIG. 4(b) is used, it ispossible to decrease the soft decision processing of a large calculationquantity, and realize satisfactory transmission characteristics, evenwhen the constellation increases along the increase in the multi-valuemodulation system. This becomes possible based on the provision of theturbo decoder that carries out a soft decision processing to the lowertwo bits of the received signal that have a possibility ofcharacteristic degradation and an error correction according to theReed-Solomon code, and the decision units that carry out a hard decisionof other bits of the received signal.

Further, it becomes possible to decrease the iteration number, byestimating the transmission data with the first R/S decoder 30 and thesecond R/S decoder 32. As a result, it becomes possible to furtherdecrease the soft decision processing of a large calculation quantityand the processing time. Further, it has been known well that, in thetransmission route that has a coexistence of random errors and bursterrors, it is possible to obtain excellent transmission characteristicsby using both the R-S (Reed-Solomon) code that corrects errors in asymbol unit, and other known error correction code.

The above explains the basic operation of the communication apparatusthat employs the multi-carrier modulation/demodulation system, and theoperation of the communication apparatus that uses a turbo code in orderto obtain satisfactory transmission characteristics andhigh-transmission rate. Next, from the viewpoint of “further improvementin the transmission rate”, there will be explained a communicationapparatus that makes a maximum utilization of the “good transmissionefficiency” and the “flexibility of functions” that are thecharacteristics of the multi-carrier modulation/demodulation system,with reference to FIG. 1. For the sake of convenience of explanation,128 sub-carriers are assumed. For example, the 256 complex FFT will beused for demodulating the 128 sub-carriers. The 128complex FFT will beused to demodulate only 64 even sub-carriers out of the 128 sub-carries.

Assume data communications according to 128 sub-carriers, based on theutilization of the DMT modulation/demodulation system. In this case, theformer half section and the latter half section of even sub-carriershave the same waveforms. The combined waveforms are also the same forthe former half section and the latter half section (refer to FIG.6(a)). On the other hand, the waveforms are inverted between the formerhalf section and the latter half section of odd sub-carriers. Thecombined waveforms are also inverted between the former half section andthe latter half section (refer to FIG. 6(b)). FIG. 6 shows diagrams thatare waveforms of sub-carriers and combined waveforms respectively.

In the transmission system of the present embodiment, when the BPSK isemployed as a primary modulation system, an inverse Fourier transform(256 complex FFT) is executed to the signal after the BPSK modulation,and a transmission symbol is generated. By utilizing the abovecharacteristics, the transmission symbol is prepared as a half symbolwithout changing the number of bits allocated to the sub-carriers. Thetransmission rate is improved in this way. However, when thetransmission symbol is prepared as a half symbol, it is not possible tomaintain orthogonality of the OFDM symbol, and this brings about mutualinterference between sub-carriers.

On the other hand, the reception system separates the received signalinto even sub-carriers and odd sub-carriers, thereby to make it possibleto demodulate the sub-carriers when interference has occurred.Specifically, the reception system first demodulates only evensub-carriers, and thereafter demodulates odd sub-carriers.

The operation of the reception system will be explained next. First, the128 complex FFT 2 receives a digital waveform (half-symbolized receivedsymbol) after a filtering processing and an A/D conversion, and convertsa serial data into a parallel data. The 128 complex FFT 2 executes aFourier transform to the parallel data. In other words, the 128 complexFFT 2 extracts only 64 even sub-carriers from among 128 sub-carriers.Usually, a full 256 complex FFT is used to perform Fourier transform ona received signal (128 sub-carriers). However, in order to performFourier transform on only the even sub-carrier component of thehalf-symbolized received symbol in this case, the 128 complex FFT thatis a half of the 256 complex FFT is used. The odd sub-carriers cannotmaintain orthogonality, and thereby become noise.

Next, the decoder 3 receives extracted 64 even sub-carriers, and carriesout the decoding according to the predetermined method (refer to FIG.4(b)). After making a decision, the decoder 3 reproduces the originaltransmission data. The data allocated to the even sub-carriers is outputas it is.

Further, in the reception system, the 128 complex IFFT 4 carries out aninverse fast Fourier transform to the data that has been allocated tothe even sub-carriers, and thereby generates a symbol that has beenstructured with only the waveform of the even sub-carriers (refer toFIG. 6(a)).

Next, the subtractor 5 removes the symbol component that has beenstructured with only the waveform of the even sub-carriers, from thehalf-symbolized received symbol, and extracts a symbol (half symbol)that has been structured with only the waveform of odd sub-carriers(refer to FIG. 6(b)). The symbol generating section 6 adds a symbol thathas been generated by copying and inverting the symbol, to the back ofthe symbol after the subtraction by utilizing the characteristics of theodd sub-carriers shown in FIG. 6(b). The symbol generating section 6generates a symbol in the status before the transmission system executesthe half-symbolization.

Last, in the reception system, the 256 complex FFT 7 executes a Fouriertransform to the received symbol (full symbol) that has been structuredwith only the odd sub-carriers. The decoder 8 receives extracted 64 oddsub-carriers, and carries out the decoding according to thepredetermined method (refer to FIG. 4(b)). After making a decision, thedecoder 8 reproduces the original transmission data.

It is noted that, in the present embodiment, when an error has occurredin the decoded data, it is possible to improve the demodulationcharacteristics by repeatedly executing the following processing. The256 complex IFFT 9 carries out an inverse fast Fourier transform to thedata that has been allocated to the odd sub-carriers. The symbolgenerating section 10 prepares a half-rated time waveform of the oddsub-carriers after the inverse fast Fourier transform by utilizing thecharacteristics of the even sub-carriers shown in FIG. 6(a)). As aresult, it is possible to generate a symbol (half symbol) that has beenstructured with only the odd sub-carriers (refer to FIG. 6(a)). Next,the subtractor 1 removes the symbol component that has been structuredwith only the waveform of the odd sub-carriers, from the receivedsymbol. Thereafter, the reception system carries out a demodulationprocessing by using the received symbol after the removal of the symbolcomponent.

A reason why it is possible to individually demodulate the evensub-carriers and the odd sub-carriers by separating these sub-carrierswill be explained next.

In general, in the OFDM modulator, a combined waveform of a plurality ofsub-carriers becomes an OFDM modulation wave as shown in the equation(1). $\begin{matrix}{{s(t)} = {R\quad {e\left\lbrack {\sum\limits_{n = 0}^{N - 1}{d_{n}^{{j2\pi}\quad n\quad f_{0}t}}} \right\rbrack}}} & (1)\end{matrix}$

where Re [ ] represents a real part, d_(n)=R_(n)+jI_(n), and 0≦t≦T_(s)(T_(s) represents an OFDM symbol period), f₀ represents a carrierinterval between adjacent sub-carriers, and nf₀ represents an n-thsub-carrier.

When a complex equalization low-pass signal of the OFDM is expressed asu (t), this u (t) can be expressed as given in the equation (2).$\begin{matrix}{{u(t)} = {\sum\limits_{n = 0}^{N - 1}{d_{n}^{{j2\pi}\quad n\quad f_{0}t}}}} & (2)\end{matrix}$

When u (t) is sampled for each 1/(Nf₀), it is possible to express thesampled signal u (k/Nf₀) as given in the equation (3). $\begin{matrix}{\begin{matrix}{{u\frac{k}{N\quad f_{0}}} = {\sum\limits_{n = 0}^{N - 1}{d_{n}^{\frac{{j2\pi}\quad n\quad f_{0}k}{N\quad f_{0}}}}}} \\{= {\sum\limits_{n = 0}^{N - 1}{d_{n}^{{\frac{j2\pi}{N}\quad}^{n\quad k}}}}}\end{matrix}} & (3)\end{matrix}$

From the equation (3), it is possible to express the OFDM modulationwave u (k/Nf₀) of even sub-carriers as given in the equation (4), wheren=2i (i=0, 1, 2, . . . , (N/2)−1). $\begin{matrix}{\begin{matrix}{{u\left( \frac{k}{N\quad f_{0}} \right)} = {\sum\limits_{n = {{2i} = 0}}^{N - 2}{d_{n}\left( ^{\frac{j2\pi}{N}} \right)}^{n\quad k}}} \\{= {\sum\limits_{n = {{2i} = 0}}^{N - 2}{d_{2i}\left( ^{\frac{j2\pi}{N}} \right)}^{2i\quad k}}}\end{matrix}} & (4)\end{matrix}$

When the equation (4) is substituted by the equation (5) by settingk=(N/2) a+b (a=0, 1, b=0, 1, . . . , N/2), it is possible to express theOFDM modulation wave of even sub-carriers as given in the equation (6).$\begin{matrix}{\quad {{W_{N}^{i} = ^{\frac{{j2\pi}\quad i}{N}}}\quad {{W_{N}^{i + N} = W_{N}^{i}},{W_{N}^{i + {N/2}} = {- W_{N}^{i}}}}}} & (5)\end{matrix}$

$\begin{matrix}{{u\left( \frac{{\frac{N}{2}a} + b}{N\quad f_{0}} \right)} = \left\{ \begin{matrix}{{a = 0},{\sum\limits_{n = {{2i} = 0}}^{N - 2}{d_{2i}W_{N}^{2b\quad i}}}} & \left( {f\quad o\quad r\quad m\quad e\quad r\quad h\quad a\quad l\quad f{\quad \quad}s\quad e\quad c\quad t\quad i\quad o\quad n} \right) \\{{a = 1},{\sum\limits_{n = {{2i} = 0}}^{N - 2}{d_{2i}W_{N}^{2b\quad i}}}} & \left( {l\quad a\quad t\quad t\quad e\quad r\quad h\quad a\quad l\quad f{\quad \quad}s\quad e\quad c\quad t\quad i\quad o\quad n} \right)\end{matrix} \right.} & (6)\end{matrix}$

From the equation (6), it is known that the former half section and thelatter half section of the even sub-carriers have the same waveforms.

On the other hand, it is possible to express the OFDM modulation wave u(k/Nf₀) of odd sub-carriers as given in the equation (7), where n=21+1(1=0, 1, 2, . . . , (N/2)−1). $\begin{matrix}{\begin{matrix}{{u\left( \frac{k}{N\quad f_{0}} \right)} = {\sum\limits_{n = 0}^{N - 1}{d_{n}\left( ^{\frac{j2\pi}{N}} \right)}^{n\quad k}}} \\{= {\sum\limits_{n = {{{2l} + 1} = 1}}^{N - 1}{d_{({{2l} + 1})}\left( ^{\frac{j2\pi}{N}} \right)}^{{({{2l} + 1})}\quad k}}}\end{matrix}} & (7)\end{matrix}$

Further, when the equation (7) is substituted by the equation (8) bysetting k=(N/2) a+b (a=0, 1, b=0, 1, . . . , (N/2)−1), it is possible toexpress the OFDM modulation wave of odd sub-carriers as given in theequation (9). $\begin{matrix}{\quad {{W_{N}^{1} = ^{\frac{{j2\pi}\quad 1}{N}}}\quad {{W_{N}^{1 + N} = W_{N}^{1}},{W_{N}^{1 + {N/2}} = {- W_{N}^{1}}}}}} & (8) \\{{u\left( \frac{{\frac{N}{2}a} + b}{N\quad f_{0}} \right)} = \left\{ \begin{matrix}{{a = 0},{\sum\limits_{n = {{{2l} + 1} = 1}}^{N - 1}{d_{({{2l} + 1})}W_{N}^{{({{2l} + 1})}b}}}} & \left( {f\quad o\quad r\quad m\quad e\quad r\quad h\quad a\quad l\quad f{\quad \quad}s\quad e\quad c\quad t\quad i\quad o\quad n} \right) \\{{a = 1},{- {\sum\limits_{n = {{{2l} + 1} = 1}}^{N - 1}{d_{({{2l} + 1})}W_{N}^{{({{2l} + 1})}b}}}}} & \left( {l\quad a\quad t\quad t\quad e\quad r\quad h\quad a\quad l\quad f{\quad \quad}s\quad e\quad c\quad t\quad i\quad o\quad n} \right)\end{matrix} \right.} & (9)\end{matrix}$

From the equation (9), it is known that the former half section and thelatter half section of the even sub-carriers have the invertedwaveforms.

Therefore, by extracting only the former half sections from the equation(6) and the equation (9), it is possible to express as shown in theequation (10). $\begin{matrix}\begin{matrix}{\quad {{u\left( \frac{{\frac{N}{2}a} + b}{N\quad f_{0}} \right)} = {\sum\limits_{n = 1}^{N - 1}{d_{n}W_{N}^{n\quad b}}}}} & \quad \\{\quad {{u\left( \frac{b}{N\quad f_{0}} \right)} = {\sum\limits_{n = 1}^{N - 1}{d_{n}W_{N}^{n\quad b}}}}} & \left( {{f\quad o\quad r\quad m\quad e\quad r\quad h\quad a\quad l\quad f{\quad \quad}s\quad e\quad c\quad t\quad i\quad o\quad n\text{:}\quad a} = 0} \right)\end{matrix} & (10)\end{matrix}$

Next, a signal component of even sub-carriers is extracted from theequation (10). When an odd sub-carrier component of d_(n) is 0, a signalcomponent of even sub-carriers becomes the same as that of the formerhalf section of the equation (6), and becomes as shown in the equation(11). $\begin{matrix}\begin{matrix}{\quad {{u\left( \frac{b}{N\quad f_{0}} \right)} = {\sum\limits_{i = 0}^{{N/2} - 1}{d_{2i}W_{N}^{2i\quad b}}}}} & \left( {f\quad o\quad r\quad m\quad e\quad r\quad h\quad a\quad l\quad f\quad s\quad e\quad c\quad t\quad i\quad o\quad n} \right)\end{matrix} & (11)\end{matrix}$

When the signal component of the even sub-carriers is demodulated withthe FFT of the N/2 input, it is possible to express the modulatedsignaly (2k) as shown in the equation (12), (k=0, 1, . . . , (N/2)−1).$\begin{matrix}{\begin{matrix}{{y\left( {2k} \right)} = {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{{u\left\lbrack \frac{b}{N\quad f_{0}} \right\rbrack}W_{N/2}^{{- b}\quad k}}}}} \\{= {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{i = 0}^{{N/2} - 1}{{d2i}\quad W_{N}^{2i\quad b}W_{N/2}^{{- b}\quad k}}}}}} \\{= {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{i = 0}^{{N/2} - 1}{{d2i}\quad W_{N/2}^{i\quad b}W_{N/2}^{{- b}\quad k}}}}}} \\{= {\frac{1}{N/2}{\sum\limits_{i = 0}^{{N/2} - 1}{d_{2i}{\sum\limits_{b = 0}^{{N/2} - 1}W_{N/2}^{{({i - k})}\quad b}}}}}}\end{matrix}} & (12)\end{matrix}$

A relationship of the equation (13) is established. $\begin{matrix}{{\sum\limits_{b = 0}^{{N/2} - 1}W_{N/2}^{{({i - k})}\quad b}} = \left\{ \begin{matrix}{\quad {{N/2}\left( {{{i - k} = 0},{{\pm N}/2},{{\pm 2}{N/2}},\ldots} \right)}} \\{\quad {0\quad \left( {o\quad t\quad h\quad e\quad r\quad s} \right)}}\end{matrix} \right.} & (13)\end{matrix}$

Therefore, when the relationship of the equation (13) is applied to theequation (12), it is possible to express the modulated signal as shownin the equation (14). $\begin{matrix}{{y\left( {2k} \right)} = \left\{ \begin{matrix}{\quad {d_{2k}\left( {0 \leq k < {{N/2} - 1}} \right)}} \\{\quad {o\quad m\quad i\quad t\quad t\quad e\quad d\quad \left( {o\quad t\quad h\quad e\quad r\quad k} \right)}}\end{matrix} \right.} & (14)\end{matrix}$

Next, a signal component of odd sub-carriers is extracted from theequation (10). When an even sub-carrier component of d_(n) is 0, asignal component of odd sub-carriers becomes the same as that of theformer half section of the equation (9), and becomes as shown in theequation (15). $\begin{matrix}{{u\left( \frac{b}{N\quad f_{0}} \right)} = {\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}W_{N}^{{({{2l} + 1})}^{b}}\quad \left( {{f\quad o\quad r\quad m\quad e\quad r\quad h\quad a\quad l\quad f\quad s\quad e\quad c\quad t\quad i\quad o\quad n\text{:}\quad a} = 0} \right)}}} & (15)\end{matrix}$

When the signal component of the odd sub-carriers is demodulated withthe FFT of the N/2 input, it is possible to express the modulated signaly′ (2k) as shown in the equation (16) (k=0, 1, . . . , (N/2)−1).However, y′ (2k) expresses an interference component of the oddsub-carriers. $\begin{matrix}{\begin{matrix}{{y^{\prime}\left( {2k} \right)} = \quad {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{{u\left( \frac{b}{N\quad f_{0}} \right)}W_{N/2}^{{- b}\quad k}}}}} \\{= \quad {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}W_{N}^{{({{2l} + 1})}^{b}}W_{N/2}^{{- b}\quad k}}}}}} \\{= \quad {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}W_{N/2}^{{({l + {1/2}})}^{b}}W_{N/2}^{{- b}\quad k}}}}}} \\{= \quad {\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}W_{N/2}^{{({l + {1/2} - k})}^{b}}}}}}} \\{= \quad {{\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}{\cos \left( \frac{2{\pi \left( {l + {1/2} - k} \right)}\quad b}{N/2} \right)}}}}} +}} \\{\quad {j\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}{\sin \left( \frac{2{\pi \left( {l + {1/2} - k} \right)}\quad b}{N/2} \right)}}}}}} \\{= \quad {{\frac{1}{N/2}{\sum\limits_{l = 0}^{{N/2} - 1}d_{{2l} + 1}}} +}} \\{\quad {j\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}{\sin \left( \frac{2{\pi \left( {l + {1/2} - k} \right)}\quad b}{N/2} \right)}}}}}}\end{matrix}} & (16)\end{matrix}$

Therefore, data z (2k) that is allocated to the even sub-carriersobtained after demodulating the half symbol can be expressed as shown inthe equation (17). $\begin{matrix}{\begin{matrix}{{z\left( {2k} \right)} = \quad {{y\left( {2k} \right)} + {y^{\prime}\left( {2k} \right)}}} \\{= \quad {d_{2k} + {\frac{1}{N/2}{\sum\limits_{l = 0}^{{N/2} - 1}d_{{2l} + 1}}} +}} \\{\quad {j\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{d_{{2l} + 1}{\sin \left( \frac{2{\pi \left( {l + {1/2} - k} \right)}\quad b}{N/2} \right)}}}}}}\end{matrix}} & (17)\end{matrix}$

When the BPSK of disposition of signal points expressed in the code(0, 1) and the signal (1, −1) is used, it is possible to express adecision value Z (2k) of the data z (2k) allocated to the evensub-carriers as shown in the equation (18) by judging only the realpart. However, d_(2k)=R_(2k)+jI_(2k). Particularly, d_(2k)=R_(2k) forthe BPSK. $\begin{matrix}{\begin{matrix}{{Z\left( {2k} \right)} = \quad {R\quad {e\left\lbrack {{y\left( {2k} \right)} + {y^{\prime}\left( {2k} \right)}} \right\rbrack}}} \\{= \quad {R_{2k} + {\frac{1}{N/2}{\sum\limits_{l = 0}^{{N/2} - 1}R_{{2l} + 1}}} +}} \\{\quad {j\frac{1}{N/2}{\sum\limits_{b = 0}^{{N/2} - 1}{\sum\limits_{l = 0}^{{N/2} - 1}{R_{{2l} + 1}{\sin \left\lbrack \frac{2{\pi \left( {l + {1/2} - k} \right)}\quad b}{N/2} \right\rbrack}}}}}} \\{= \quad {R_{2k} + {\frac{1}{N/2}{\sum\limits_{l = 0}^{{N/2} - 1}R_{{2l} + 1}}}}}\end{matrix}} & (18)\end{matrix}$

Therefore, in the case of the BPSK, the interference component to thedata of even sub-carriers becomes a value obtained by adding modulatedsignals of the (1/N) timed data of odd sub-carriers for the totalsub-carrier component. When the modulated data has been scrambled, it isknown that the signal allocated to the odd sub-carriers hassubstantially equal probabilities of the occurrence of 1 and theoccurrence of −1. Further, the total sum of the total odd sub-carriersbecomes close to 0. Therefore, when the number of sub-carriers issufficiently large, it is possible to approximate the equation (18) tothe equation (19). $\begin{matrix}{\begin{matrix}{{Z\left( {2k} \right)} = \quad {R_{2k} + {\frac{1}{N/2}{\sum\limits_{l = 0}^{{N/2} - 1}R_{{2l} + 1}}}}} \\{\cong \quad R_{2k}}\end{matrix}} & (19)\end{matrix}$

From the above, it can be known that it is possible to individuallydemodulate the even sub-carriers and the odd sub-carriers by separatingthese sub-carriers when an a transmission symbol is generated byexecuting an inverse Fourier transform to a signal after the BPSKmodulation, and when the transmission symbol is half-symbolized.

As explained above, according to the present embodiment, when the BPSKis employed as a primary modulation system, the communication apparatusat the transmission side transmits a signal by half-symbolizing thetransmission symbol. The communication apparatus at the reception sideseparates the received signal into even sub-carriers and oddsub-carriers, and demodulates only the received symbol of thehalf-symbolized even sub-carriers. Thereafter, the communicationapparatus at the reception side removes the symbol component of the evensub-carriers from the received symbol, and then demodulates only thereceived symbol of the odd sub-carriers. Based on this, it becomespossible to compress the symbol on the time axis, and expand thetransmission capacity to about two times. Further, according to thepresent embodiment, the symbol that is structured with only the waveformof the odd sub-carriers is fed back, and the odd sub-carriers thatbecome the noise component can be removed from the received symbol.Therefore, it is possible to substantially improve the demodulationprecision.

Although the 128 sub-carriers are assumed for the sake of convenience ofexplanation in the present embodiment, the number of sub-carriers is notlimited to this. When the number of sub-carriers is other than 128, thenumbers of the FFT and the IFFT also change corresponding to the numberof the sub-carriers.

Second Embodiment

FIG. 7 is a diagram that shows a structure of a second embodiment of thecommunication apparatus according to the present invention.Specifically, this is a diagram that shows a structure of the receptionsystem of the communication apparatus according to the presentinvention. In FIG. 7, a reference numeral 61 denotes a transmissionpattern generating section, 62 denotes an inverse fast Fouriertransforming section (256 complex IFFT), and 63 denotes a symbolgenerating section. A reference numeral 64 denotes a subtractor, 65denotes a dispersion section, 66 denotes a minimum value searchingsection, and 67 denotes a decision section. The structure and theoperation of the transmission system are similar to those of the firstembodiment, and therefore, explanation on them will be omitted. Onlysections that are different from those of the first embodiment will beexplained.

The operation of the reception system of the communication apparatusaccording to the present invention will be explained below. First, thetransmission pattern generating section 61 generates a combination ofdata that could occur, and occasionally outputs the data to the inversefast Fourier transforming section 62. The inverse fast Fouriertransforming section 62 carries out an inverse fast Fourier transform tothe received combination of data, and generates a symbol that isstructured with time axis data of even sub-carriers and oddsub-carriers. The symbol generating section 63 deletes the latter halfsection of the symbol, and generates a half-symbolized symbol.

The subtractor 64 subtracts a symbol that is occasionally received fromthe symbol generating section 63, from the received symbol. Thedispersion section 65 sequentially obtains a squared average (adispersion value) of amplitude for all the received subtraction results.The minimum value searching section 66 detects a minimum value fromamong the received squared average value. The decision section 67decides a data combination corresponding to the minimum value of thesquared average value as most likely, from among all data combinationsthat could occur, and fixes this data combination as a final decisionvalue.

As explained above, according to the present embodiment, thecommunication apparatus at the transmission side executes an inverseFourier transform (256 complex FFT) to the signal after the BPSKmodulation, thereby to generate a transmission symbol and half-symbolizethis transmission symbol. The communication apparatus at the receptionside occasionally cancels the time axis data (half-symbolized symbol) ofall sub-carriers that have been generated based on the data combinationsthat could occur, from the received symbols. Further, the communicationapparatus at the reception side calculates a dispersion value by usingthe signal after the cancellation, and decides a data combinationcorresponding to the minimum value of the calculation result as adecision value. As a result, it becomes possible to compress the data onthe time axis, and enlarge the transmission capacity to about two times.

Third Embodiment

FIG. 8 is a diagram that shows a structure of a third embodiment of thecommunication apparatus according to the present invention.Specifically, this is a diagram that shows a structure of the receptionsystem of the communication apparatus according to the presentinvention. In FIG. 8, a reference numeral 71 denotes a symbol generatingsection, 72 denotes a fast Fourier transforming section (N value complexFFT), 73 denotes an even-carrier generating section, and 74 denotes anodd-carrier generating section. The structure and the operation of thetransmission system are similar to those of the first embodiment, andtherefore, explanation on them will be omitted. Only sections that aredifferent from those of the first or second embodiment will beexplained.

The operation of the reception system of the communication apparatusaccording to the present invention will be explained below. First, thesymbol generating section 71 adds a symbol of all 0 to the back of ahalf-symbolized received symbol. The transmission system generates asymbol of a size before the transmission system executes the halfsymbolization. The N value complex FFT 72 carries out a Fouriertransform to a symbol that has been generated by the symbol generatingsection 71, and extracts/outputs even sub-carriers and odd sub-carriers.

For example, when N is 4, it is possible to express the evensub-carriers and odd sub-carriers (y (0), y (1) y (3), y (4)) as shownin the equation (20). $\begin{matrix}{\begin{bmatrix}{y(0)} \\{y(1)} \\{y(2)} \\{y(3)}\end{bmatrix} = {{{\frac{1}{N}\begin{bmatrix}W_{4}^{0} & W_{4}^{0} & W_{4}^{0} & W_{4}^{0} \\W_{4}^{0} & W_{4}^{- 1} & W_{4}^{- 2} & W_{4}^{- 3} \\W_{4}^{0} & W_{4}^{- 2} & W_{4}^{- 4} & W_{4}^{- 6} \\W_{4}^{0} & W_{4}^{- 3} & W_{4}^{- 6} & W_{4}^{- 9}\end{bmatrix}}\quad\begin{bmatrix}W_{4}^{0} & W_{4}^{0} & W_{4}^{0} & W_{4}^{0} \\W_{4}^{0} & W_{4}^{1} & W_{4}^{2} & W_{4}^{3} \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}}\quad\begin{bmatrix}{d(0)} \\{d(1)} \\{d(2)} \\{d(3)}\end{bmatrix}}} & (20)\end{matrix}$

The above explains the case of N=4, that is, a four-value FFT. As ageneral expression method, it is possible to express the N value complexFFT 72 as shown in the equation (21). $\begin{matrix}{\frac{1}{N}\begin{bmatrix}W_{N}^{0} & W_{N}^{0} & W_{N}^{0} & \ldots & W_{N}^{0} \\W_{N}^{0} & W_{N}^{- 1} & W_{N}^{- 2} & \ldots & W_{N}^{- {({N - 1})}} \\W_{N}^{0} & W_{N}^{- 2} & W_{N}^{- 4} & \ldots & W_{N}^{{- 2}{({N - 1})}} \\W_{N}^{0} & W_{N}^{- 3} & W_{N}^{- 6} & \ldots & W_{N}^{{- 3}{({N - 1})}} \\\vdots & \vdots & \vdots & \quad & \vdots \\W_{N}^{0} & W_{N}^{- {({N - 1})}} & W_{N}^{{- 2}{({N - 1})}} & \ldots & W_{N}^{- {({N - 1})}^{2}}\end{bmatrix}} & (21)\end{matrix}$

Next, the even-carrier decision section 73 judges a data allocated tothe received even sub-carriers, and reproduces the original transmissiondata. The odd-carrier decision section 74 judges a data allocated to thereceived odd sub-carriers, and reproduces the original transmissiondata.

As explained above, according to the present embodiment, thecommunication apparatus at the transmission side executes an inverseFourier transform (256 complex FFT) to the signal after the BPSKmodulation, thereby to generate a transmission symbol and half-symbolizethis transmission symbol. The communication apparatus at the receptionside adds a symbol of all 0 to the back of the half-symbolized receivedsymbol. The communication apparatus at the reception side furthercarries out a Fourier transform to a symbol after the addition of thesymbol of all 0 thereto, and extracts even sub-carriers and oddsub-carriers. As a result, it becomes possible to compress the symbol onthe time axis and enlarge the transmission capacity to about two times.It is noted that, in the present embodiment, an error correction sectionlike the R-S (Reed-Solomon) may be provided at a latter stage of eachdecision section in order to improve the demodulation precision.

Fourth Embodiment

FIG. 9 is a diagram that shows a structure of a fourth embodiment of thecommunication apparatus according to the present invention.Specifically, this is a diagram that shows a structure of the receptionsystem of the communication apparatus according to the presentinvention. In FIG. 9, a reference numeral 81 denotes an even-carrierdecision section, 82 denotes an even-carrier interference componentcalculation section, 83 denotes an odd-carrier decision section, 84denotes an odd-carrier interference component calculation section, and85 and 86 denote subtractors. Structures that are similar to those ofthe above embodiments are attached with the same reference numerals, andexplanation on them will be omitted. Only sections that are differentfrom those of the third embodiment will be explained.

The operation of the transmission system and the reception system of thecommunication apparatus according to the present invention will beexplained below. The transmission system of the present embodimentexecutes an inverse Fourier transform (256complex FFT) to a signal aftera QPSK modulation, thereby to generate a transmission symbol. Thetransmission system half-symbolizes the transmission symbol withoutchanging the number of bits allocated to sub-carriers by utilizing thecharacteristics shown in FIG. 6 to improve the transmission rate.However, when the transmission symbol is half-symbolized, it becomesimpossible to maintain the orthogonality of the OFDM symbol, and amutual interference occurs between the sub-carriers.

On the other hand, in the reception system of the present embodiment,the even-carrier decision section 81 makes a hard decision on the dataof even carriers that are output of an N value complex FFT 72. Further,the even-carrier interference component calculation section 82calculates the interference component of the even carriers, that is, thenoise component of the odd carriers.

The subtractor 86 removes the interference component of the evencarriers from the odd sub-carrier output of the N value complex FFT 72.The odd-carrier decision section 83 makes a hard decision on the data ofodd sub-carriers after the removal of the interference component.Further, the odd-carrier interference component calculation section 84calculates the interference component of the odd carriers, that is, thenoise component of the even carriers.

The subtractor 86 removes the interference component of the odd carriersfrom the even sub-carrier output of the N value complex FFT 72.Thereafter, the reception system of the present embodiment outputs adecision result of the even-carrier decision section 81 and a decisionresult of the odd-carrier decision section 83 after repeating the aboveprocessing by a predetermined number, as final decision values.

As explained above, according to the present embodiment, thecommunication apparatus at the transmission side executes an inverseFourier transform (256 complex FFT) to the signal after the QPSKmodulation, thereby to generate a transmission symbol and half-symbolizethis transmission symbol. The communication apparatus at the receptionside adds a symbol of all 0 to the back of the half-symbolized receivedsymbol. The communication apparatus at the reception side furthercarries out a Fourier transform to a symbol after the addition of thesymbol of all 0 thereto, and extracts even sub-carriers and oddsub-carriers. The communication apparatus at the reception side outputsthe decision results after the removal of mutual interference as finaldecision values. As a result, it becomes possible to compress the symbolon the time axis and enlarge the transmission capacity to about twotimes, when the QPSK is employed as a primary modulation system. It isnoted that, in the present embodiment, it is possible to provide anerror correction section like the R-S (Reed-Solomon) at a latter stageof each decision section, in order to improve the demodulationprecision.

As explained above, according to the present invention, an inverseFourier transform (256 complex FFT) is executed to the signal after theBPSK modulation to generate a transmission symbol, and this transmissionsymbol is half-symbolized. The communication apparatus at the receptionside separates the received signal into even sub-carriers and oddsub-carriers, and demodulates only the received symbol of thehalf-symbolized even sub-carriers. Thereafter, the communicationapparatus at the reception side removes the symbol component of the evensub-carriers from the received symbol, and then demodulates only thereceived symbol of the odd sub-carriers. Based on this, it becomespossible to compress the symbol on the time axis, and there is an effectthat it is possible to obtain the communication apparatus that canexpand the transmission capacity to about two times.

According to the next invention, the symbol that is structured with onlythe waveform of the odd sub-carriers is fed back, and the oddsub-carriers that become the noise component can be removed from thereceived symbol. Therefore, there is an effect that it is possible toobtain the communication apparatus that can substantially improve thedemodulation precision.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the BPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side occasionally cancels thetime axis data (half-symbolized symbol) of all sub-carriers that havebeen generated based on the data combinations that could occur, from thereceived symbols. Further, the communication apparatus at the receptionside calculates a dispersion value by using the signal after thecancellation, and decides a data combination corresponding to a minimumvalue of the calculation result as a decision value. As a result, itbecomes possible to compress the data on the time axis, and there is aneffect that it possible to obtain the communication apparatus that canenlarge the transmission capacity to about two times.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the BPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side adds a symbol of all 0 tothe back of the half-symbolized received symbol. The communicationapparatus at the reception side further carries out a Fourier transformto a symbol after the addition of the symbol of all 0 thereto, andextracts even sub-carriers and odd sub-carriers. As a result, it becomespossible to compress the symbol on the time axis, and there is an effectthat it is possible to obtain the communication apparatus that canenlarge the transmission capacity to about two times.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the QPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side adds a symbol of all 0 tothe back of the half-symbolized received symbol. The communicationapparatus at the reception side further carries out a Fourier transformto a symbol after the addition of the symbol of all 0 thereto, andextracts even sub-carriers and odd sub-carriers. The communicationapparatus at the reception side outputs the decision results after theremoval of mutual interference as final decision values. As a result, itbecomes possible to compress the symbol on the time axis, and there isan effect that it is possible to obtain the communication apparatus thatcan enlarge the transmission capacity to about two times, when the QPSKis employed as a primary modulation system.

According to the next invention, an inverse Fourier transform (256complex FFT) is executed to the signal after the BPSK modulation togenerate a transmission symbol, and this transmission symbol ishalf-symbolized. The reception unit at the reception side separates thereceived signal into even sub-carriers and odd sub-carriers, anddemodulates only the received symbol of the half-symbolized evensub-carriers. Thereafter, the reception unit removes the symbolcomponent of the even sub-carriers from the received symbol, and thendemodulates only the received symbol of the odd sub-carriers. Based onthis, it becomes possible to compress the symbol on the time axis, andthere is an effect that it is possible to expand the transmissioncapacity to about two times.

According to the next invention, the symbol that is structured with onlythe waveform of the odd sub-carriers is fed back, and the oddsub-carriers that become the noise component can be removed from thereceived symbol. Therefore, there is an effect that it is possible tosubstantially improve the demodulation precision.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the BPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side occasionally cancels thetime axis data (half-symbolized symbol) of all sub-carriers that havebeen generated based on the data combinations that could occur, from thereceived symbols. Further, the communication apparatus at the receptionside calculates a dispersion value by using the signal after thecancellation, and decides a data combination corresponding to a minimumvalue of the calculation result as a decision value. As a result, itbecomes possible to compress the symbol on the time axis, and there isan effect that it possible to enlarge the transmission capacity to abouttwo times.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the BPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side adds a symbol of all 0 tothe back of the half-symbolized received symbol. The communicationapparatus at the reception side further carries out a Fourier transformto a symbol after the addition of the symbol of all 0 thereto, andextracts even sub-carriers and odd sub-carriers. As a result, it becomespossible to compress the symbol on the time axis, and there is an effectthat it is possible to enlarge the transmission capacity to about twotimes.

According to the next invention, the communication apparatus at thetransmission side executes an inverse Fourier transform (256 complexFFT) to the signal after the QPSK modulation, thereby to generate atransmission symbol and half-symbolize this transmission symbol. Thecommunication apparatus at the reception side adds a symbol of all 0 tothe back of the half-symbolized received symbol. The communicationapparatus at the reception side further carries out a Fourier transformto a symbol after the addition of the symbol of all 0 thereto, andextracts even sub-carriers and odd sub-carriers. The communicationapparatus at the reception side outputs the decision results after theremoval of mutual interference as final decision values. As a result, itbecomes possible to compress the symbol on the time axis, and there isan effect that it is possible to enlarge the transmission capacity toabout two times when the QPSK is employed as a primary modulationsystem.

Industrial Applicability

As explained above, the communication apparatus and the communicationmethod according to the present invention are suitable for datacommunications using existing communication lines based on the DMT(Discrete Multi Tone) modulation/demodulation system or the OFDM(Orthogonal Frequency Division Mutiplex) modulation/demodulation system.

What is claimed is:
 1. A communication apparatus that employs amulti-carrier modulation/demodulation system, comprising: a transmissionunit which generates a symbol to be transmitted by carrying out aninverse Fourier transform to a signal after a BPSK modulation, andtransmits the transmission symbol in a half-symbolized status, and areception unit which receives the half symbolized symbol, the receptionunit includes; a first fast Fourier transform section that carries out apredetermined Fourier transform to the half-symbolized received symbolin order to extract even sub-carriers; and a first decoder thatdemodulates data allocated to the even sub-carriers extracted by thefirst fast Fourier transform section; an inverse fast Fourier transformsection that carries out an inverse Fourier transform on the dataallocated to the even sub-carriers outputted by the first decoder, andgenerates a first symbol that is structured with a time waveform of theeven sub-carriers; a subtractor that removes a component of the firstsymbol generated by the inverse fast Fourier transform section,component from the half-symbolized received symbol, and extracts fromthe half-symbolized symbol a second symbol that is structured with atime waveform of odd sub-carriers; a symbol generating section thatgenerates a third symbol by adding a generated symbol, obtained bycopying and inverting the second symbol extracted by the subtractor, theend of the second symbol; a second fast Fourier transform section thatcarries out a predetermined Fourier transform on the third symbolgenerated by the symbol generating section in order to extract the oddsub-carriers; and a second decoder that demodulates data allocated tothe odd sub-carriers extracted by the second fast Fourier transformsection.
 2. The communication apparatus according to claim 1, whereinsaid reception unit further includes: a second inverse fast Fouriertransform section that carries out an inverse Fourier transform to thedata allocated to the odd sub-carriers outputted by the second decoder;and a second symbol generating section that generates a fourth symbolthat is structured with a time waveform of the odd sub-carriers andfeeds back the fourth symbol to a second subtractor, wherein the secondsubtractor removes a component of the fourth symbol from the halfsymbolized symbol, for enabling a demodulation processing using the halfsymbolized symbol with the component of the fourth symbol removed.
 3. Acommunication method of employing a multi-carriermodulation/demodulation system, comprising the steps of: generating asymbol to be transmitted by carrying out an inverse Fourier transform toa signal after a BPSK modulation, and transmitting the symbol in ahalf-symbolized status; receiving the half-symbolized symbol at areception unit; an demodulating even sub carries by carrying out apredetermined first Fourier transform on the half-symbolized symbol inorder to extract the even sub-carriers; demodulating data allocated tothe even sub-carriers; generating a first symbol by carrying out aninverse Fourier transform on the data allocated to the evensub-carriers, and generating the first symbol that is structured with atime waveform of the even sub-carriers; generating a second symbol byremoving the first symbol component from the half-symbolized symbol, andextracting from the half-symbolized symbol the second symbol that isstructured with a time waveform of odd sub-carriers; generating a thirdsymbol by adding a generated symbol, obtained by copying and invertingthe second symbol, to the end of the second symbol; demodulating oddsub-carriers by carrying out a predetermined second Fourier transform onthe third symbol in order to extract the odd sub-carriers; anddemodulating data allocated to the odd sub-carriers.
 4. Thecommunication method according to claim 3, further comprising the stepsof: generating a fourth symbol by carrying out an inverse Fouriertransform on the data allocated to the odd sub-carriers; and generatinga fourth symbol that is structured with a time waveform of the oddsub-carriers; and removing a component of the fourth symbol from thehalf-symbolized symbol, wherein a demodulation process is carried out byusing the half-symbolized symbol with the component of the fourth symbolremoved.